1. Field of the Invention
The present invention relates to an optical selector and a method for selecting ATM cells having a frequency of arrival, f, less than or equal to a predetermined frequency.
2. Technical Background
At the present time, in telecommunications networks, optical technology is predominantly used for the transmission of optical signals in which use is made of the known wideband properties offered by optical fibres, while the operations of processing the optical signals, such as multiplication and switching, are carried out by means of opto-electronic devices.
However, opto-electronic devices have the known disadvantages of electronic circuits which are becoming bottlenecks in optical communications systems and in optical networks. This is because electronic devices have a narrow bandwidth compared with the optical band available in optical communications systems, and are generally based on a relatively slow serial processing of the signals.
Research is therefore being increasingly directed towards the possibility of using optics not only for the transmission of signals but also for their processing. This is due to the fact that, potentially, wholly optical devices have a wide bandwidth and are transparent to the bit rate, the format and the code of the transmission.
In the switching nodes of an optical network, where multiple communication channels converge and originate, there is a widely felt need for devices capable of carrying out wholly optical operations on sequences of bits (cells).
For example, an important operation in known asynchronous transfer mode (ATM) networks, which use a method of statistical multiplexing of data packets originating from separate sources, is that of checking the frequency of the cells (peak cell rate, PCR) arriving at the input of a network.
ATM is a transmission method which consists in grouping the data transmitted from various sources into packets of digital data (cells), each of which consists of a payload of 43 bytes and a header of 5 bytes. The header comprises various fields containing various data used by the nodes of the ATM network to control the switching of the cells. This transmission method also provides a negotiation of the cell transmission frequency (PCR) before the opening of a connection between a source and an ATM network or between two different ATM networks.
The PCR check has to be carried out both at the point at which the traffic sources have access to an ATM network and at points of interconnection between different ATM networks, and consists in checking that each source or network does not generate cells at a frequency (f) greater than that negotiated, in other words greater than the frequency (PCR) which has been assigned to it by the network to which it is connected.
However, in the case to which the present description and claims relate, the expression xe2x80x9cATM networkxe2x80x9d is not used in the limiting sense but also comprises all the systems which, like the conventional ATM, are based on a negotiation of the cell frequency before the opening of a predetermined connection.
The xe2x80x9cvirtual scheduling algorithmxe2x80x9d (VSA) is one of the standard algorithms included in ITU-T Recommendation I.371, for PCR checking in ATM networks. This algorithm requires that, at the moment at which a new cell relating to a given connection between two ATM nodes arrives at one of the said access or interconnection points, a device (xe2x80x9cpolicerxe2x80x9d) capable of controlling the traffic in this network calculates the time elapsed since the transit of the last cell and that, if this time is less than a time TPCR equal to the inverse of the PCR (TPCR=1/PCR), the new cell must be rejected.
There are known electronic devices for implementing the VSA.
U.S. Pat. No. 5,541,913 describes a PCR checking device comprising a plurality of processors in parallel, each of which implements a modified virtual scheduling algorithm and thus generates conformity signals for a cell for which an activation circuit has been activated. From these conformity signals, a plurality of which may be provided for each cell of the incoming ATM flow, an arbitration module generates a discharge signal, which indicates whether this cell must or must not be included in an output flow from a discharge circuit, and feedback signals, which indicate to a corresponding processor whether or not to carry out an update, already provided, of the specified arrival time used in the aforesaid algorithm. The architecture of the device permits the implementation of any check method by means of an adaptation of the arbitration module and of the activation circuit.
Man-Yeong Jeon et al. [xe2x80x9cImplementation of a Peak Cell Rate Policer using the Virtual Scheduling Algorithmxe2x80x9d, Proceedings of IEEE ICC ""96, Dallas, Tex., USA, p. 762-766 (1996)] describe the architecture of a checking device which implements the VSA to check the PCR and discuss some problems relating to its implementation. They also illustrate some experimental results obtained for various values of PCR, to demonstrate that the device carries out its checking function in a satisfactory way.
However, there are no known optical devices for implementing a policer for checking the PCR in ATM networks.
A first aspect of the present invention is therefore an optical selector for an ATM optical network, the said selector being capable of selecting ATM cells of bits having a frequency of arrival, f, less than or equal to a predetermined frequency (PCR), the said optical selector comprising:
an input having a first device capable of blocking the entry of bits;
an output having a second device capable of permitting the outflow of bits;
an ATM cell recognition unit constructed in such a way that it recognizes the headers of ATM cells and connected to the said input and to the said output for the transit of bits from the said input to the said output; and
an optical decision unit connected operationally to the said first device, to the said second device and to the said ATM cell recognition unit.
In the course of the present description and of the attached claims, the expression xe2x80x9coptical decision unitxe2x80x9d is used to indicate a device which, having received an optical impulse at its input., is capable of supplying two optical impulses at the output, for example two optical control impulses, in which each output optical impulse has a predetermined temporal phase shift with respect to an input optical impulse and also having a predetermined duration.
Preferably, the said first device consists of a first optical gate. More preferably, the said first optical gate is in the open state when it is in normal operating conditions.
Preferably, the said second device consists of a second optical gate. More preferably, the said second optical gate is in the closed state when it is in normal operating conditions.
Advantageously, the said ATM cell recognition unit is an optical device capable of sending an optical impulse to the said optical decision unit when it recognizes the header of an ATM cell.
Preferably, the said optical decision unit processes the said optical impulse to supply a first optical control signal for the said first device, the said first optical control signal being capable of changing the state of the said first optical gate from open to closed for a time substantially equal to T, starting from when a time substantially corresponding to the duration TC of an ATM cell has elapsed since the recognition of the header of an ATM cell. Additionally, the said optical decision unit processes the said optical impulse to supply a second optical control signal for the said second device, the said second optical control signal being capable of changing the state of the said second optical gate from closed to open for a time substantially equal to the duration TC of an ATM cell, starting from the time of recognition of the header of an ATM cell by the said ATM cell recognition unit.
Preferably, the said optical decision unit also comprises a first output for the said first optical control signal connected to the said first device. More preferably, the said optical decision unit also comprises a second output for the said second optical control signal connected to the said second device.
A second aspect of the present invention is therefore a method for selecting ATM cells of bits having an arrival frequency, f, less than or equal to a predetermined frequency (PCR), the said method comprising:
a) the input of bits through suitable open input means,
b) the passage of the said bits through an ATM cell recognition unit capable of recognizing the headers of ATM cells, towards suitable closed output means;
c) the sending of a command, when the header of an ATM cell is recognized by the said ATM cell recognition unit,
i) for the opening of the said output means for a time substantially equal to the duration (TC) of an ATM cell and, at the end of this time TC,
ii) for the closing of the said input means for a time
T=1/PCRxe2x88x92TC
xe2x80x83d) the maintenance of the initial state, if the header of an ATM cell is not recognized by the said ATM cell recognition unit, in such a way that the said input means remain open for the input of an ATM cell and the said output means remain closed, thus eliminating the bits which have entered and whose header has not been recognized.
Preferably, the step c) also comprises providing a first optical control impulse for the said output means and a second optical control impulse for the said input means.
Advantageously, the said first optical control impulse has a duration substantially equal to TC, starting from the moment of recognition of the header of an ATM cell.
More advantageously, the said second optical control impulse has a duration substantially equal to T, starting from the moment when a time substantially equal to TC has elapsed since the recognition of the header of an ATM cell.